1. Field of the Invention
The present invention relates generally to a matching network for matching an antenna to a transmitter or receiver, and particularly to a dynamic matching network for multi-band antennas.
2. Description of the Related Art
Wireless communication units are nowadays used in a plurality of applications, to allow communication between a wireless means and a stationary means or also between several wireless means.
One of the most important characteristics of modern wireless communication units is their operating time, which is divided into the so-called “talk time” and the so-called “stand time”. The mentioned characteristics are influenced by antenna matching in a front end or high-frequency part, respectively. Bad antenna matching has the effect that reflections occur at the output of a power amplifier before the antenna. The reflections can also be considered as losses and reduce the (effective) transmitting power of the unit.
Larger changes of the environmental conditions of the antenna have the effect that the antenna impedance deviates from a static optimum value or a design value, respectively. Frequently, a deviation of the antenna impedance from a set value or a static optimum value, respectively, is already caused by covering the antenna with a hand. The loss of (effective) transmitting power resulting thereby has to be compensated, for example in GSM systems, by an increase of transmitting power to avoid reduction of the range. In highly linear systems, such as UMTS, the connection is even frequently interrupted due to the erroneous matching, in the above-mentioned case.
The linearity of the system depends also on antenna matching. Reflections at the antenna overlay the supply voltage of the output amplifier and cause an offset of the operating point. Thereby, a dependence of the amplification on environmental antenna reflections results, which is reflected in a decrease of the so-called eye diagram of the digital message signal or in an increase of the bit error rate, respectively.
In order to effect decoupling of the output amplifier from the antenna impedance and thus to generate immunity of output matching against environmental mismatches, an isolator can be used between the output amplifier and the antenna. Thus, FIG. 8 shows an output circuit of a communication unit according to the prior art. The output circuit is designated by 800 in its entirety. An amplifier or power amplifier 810, respectively, receives a high-frequency input signal 812 and amplifies the high-frequency input signal 812. Then, the amplifier 810 provides the amplified high-frequency signal 814 to an isolator 816. The isolator passes the amplified high-frequency signal 814 and provides the same to an antenna 820. Further, the isolator 816 attenuates the high-frequency power, which is reflected by the antenna 820, so that only a low measure of reflected power is applied at the output of the amplifier 810. Thus, the amplifier 810 is decoupled from reflections occurring at the antenna 820, to a certain amount, in dependence on the isolation characteristics of the isolator 816. As a result, the amplifier 810 can operate, for example in an operating range, which is as linear as possible. Power reflected at the antenna 820 is dissipated at least partly in the isolator 816. The antenna circuit of FIG. 8, consisting of an output amplifier and an isolator allows thus at least partly decoupling of the amplifier 810 from the antenna characteristics.
Isolators are basically used in SMD technique (SMD=surface mounted devices) with a size of generally more than 0805 (L×W×H=length×width×height: 2 mm×1.25 mm=1.35 mm). Thus, the isolators in SMD technique form a concentrated component. Compared to integrated components or common SMD devices of packet size 0201 (L×W×H=length ×width×height: 0.5 mm×0.25 mm×0.3 mm), a series of disadvantages is accepted with isolators. Thus, isolators have typically comparatively large space requirements, wherein particularly the additional height of approximately one millimeter has a spurious effect. Further, the isolators have additional weight and cause transmission losses. In addition to the above-mentioned disadvantages, in isolators, there are further two other disadvantages of ferritic components, namely the comparatively high cost and the harmonics caused by their non-linear behavior.
Contrary to older wireless communication units, modern systems are frequently multiband-capable. Almost all commercially available mobiles are, for example, implemented for at least two frequencies (dual-band operation). However, there are mainly devices or units, respectively, on the market, which allow operation in three frequency bands (triple-band operation). Additionally, many devices include a bluetooth interface. Basically, a matched antenna has to be provided for each of the used frequency bands. Thus, additional switching networks are required to direct the transmission power to the correct antenna. These switching networks cause additional losses reducing the operating time of the system.
Further, dual-band antennas are available, which offer matching for two frequencies (e.g. for GSM 900 and GSM 1800). Thus, the number of required antennas is lower than the number of operating frequency bands.
US-2004/0185916 A1 describes a high-frequency amplifier tolerant towards load variations. The usage of a “loadvariant” amplifier offers the possibility to replace an isolator. According to the mentioned reference, an amplifier arrangement is used, which is very similar to a balanced amplifier arrangement. An input network divides an input signal and generates a phase difference of 90 degrees between two amplifiers connected in parallel. An output network compensates this phase difference and combines the power of the two amplifiers. Thereby, it is ensured that the amplifiers are coupled to a load at an output of the amplifier, once inductively and once capacitively. Variations of the load impedance are thus largely compensated for at least one of the amplifiers.
With regard to compensation of mismatchings at the antenna, the described load-variant amplifier for omitting the isolator can be compared to an ideal isolator with decoupling of 6 dB in the ideal case. Further, space and cost savings resulting by omitting the isolator, are largely eliminated by the usage of the required coupling and matching networks. Further, the above-described circuitry of a load-variant amplifier represents a solution for a single fixed frequency band.
The German patent application with the application number DE 10 2004 054442 entitled “Antennenarchitektur und Koppler” describes a so-called “ISO antenna”. The ISO antenna represents an arrangement for saving an isolator. The mentioned approach follows also a balanced amplifier arrangement. A coupling network divides an input power between two output ports, wherein the two paths have a phase difference of 90 degrees. Two antennas with identical radiation characteristics are deposited or disposed, respectively, at the output ports close to each other. A fourth port of the coupling network is terminated with a terminating resistor (having the system impedance) in a reflection-free way. Similar to an input port of a balanced amplifier arrangement, the two powers reflected at the antennas overlay with a phase difference of 180 degrees and cancel each other. Further, in the shown approach, two frequency bands can be matched simultaneously.
In comparison to the concept illustrated in US-2004/0185916 A1, the described ISO antenna provides better isolation and can be matched for two frequencies. In contrast to the approach shown in US-2004/0185916 A1, the high system requirements are shifted to the antenna level when using the ISO antenna. Thus, a second antenna with identical radiation characteristics is required, which has to be disposed close to the first antenna. By using an additional antenna, there is no effective space saving. Further, by using an ISO antenna, merely two fixed frequency bands can be matched. A further disadvantage when using an ISO antenna is that in mismatching of the antenna, the transmitting power is provided to the resistor at the fourth port of the coupling network. There, the transmitting power reflected by the antennas is converted into heat as ohmic loss. Thus, in mismatching of the antennas, the effective transmitting power is heavily reduced.
In summary, it can be said that two problems occur according to the prior art. A weakness of known antennas is their sensitivity against environmental influences. Environmental mismatchings, for example caused by larger well-conducting areas, are passed on directly to the next stages, for example an amplifier, and cannot be compensated. Correspondingly, non-linearities are forced upon the output amplifier by mismatching, or the amplifier is brought to an operating point by the transmitting power reflected by the mismatched antenna, respectively, where linear operation is no longer ensured. Further, the reflected power, which is proportional to the antenna mismatching, has to be compensated by higher power consumption. In other words, if power is reflected by a non-matched antenna, the reflected power is not radiated. In order to obtain radiated power sufficient for reception, the transmitting power generated by the transmitting amplifier has to be increased, which results in an increase of the supply power consumed by the transmitting amplifier. In the case of mismatching of the antennas, the mentioned connection results in a shorter operating runtime.
In summary, it can be said that the isolators conventionally used for decoupling the output amplifier from environmental antenna mismatchings cause relative high costs and increase the weight of the system. The transmission losses caused by the isolators have an adverse effect on the operating time of accumulators supplying a mobile communication unit.
Further mismatchings result in transmitter and receiver using a single antenna for several frequency bands. Multiband transmitters and receivers, such as mobiles or personal digital assistants (PDA) use several antennas, to be well matched for the respectively used frequency range, for example GSM 900, GMS 1800, UMTS and/or bluetooth. Since every antenna causes additional weight and cost and contributes to the overall size of the unit, frequently, dual-band antennas are used. In other words, the usage of dual-band antennas, which are matched both at 900 MHz and at 1800 MHz, represents an approach for obtaining a reduced number of antennas. However, dual functionality of an antenna is generally paid for on the expense of matching of the dual-band antenna decreased compared to single-band antennas in the individual operating frequencies.
Thus, when using dual-band antennas or multi-band antennas, there is typically worse matching than when using a single-band antenna.